High stability free piston machine



June 17, 1958 RAMSEY EIAL 2,839,035

HIGH STABILITY FREE PISTON MACHINE Filed Dec. 28, 1955 2, 2 Sheets-Sheet1 ATTORNEYS June 17, 1958 R. P. RAMSEY ETAL HIGH STABILITY FREE PISTONMACHINE 2 Sheets-Sheet 2 Filed Dec. 28, 1955 mooo now

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United States PatentOfi fice 2,839,035 Patented June 17, 1958 HIGHSTABILITY FREE PISTON MACHINE Robert Pritchard Ramsey, Mount Vernon,Ohio, and Shao Lee S00, Princeton, N. J., assignors to TheCooper-Bessemer Corporation, Mount Vernon, Ohio, a corporation of OhioApplication December 28, 1955, Serial No. 555,845

1 Claim. (Cl. 123-46) This invention relates to free piston machineryand has for its primary object the provision of a machine which isinherently stable in its operation.

Free piston machines, particularly those used either to drive gascompressors or incorporating gas compressing cylinders, are generallyincapable of continued operation in the event that the power impulseportion of the cycle Thus, if the ignition fails to fire a fuel chargewhere electric ignition is used, or a fuel injection system fails todeliver the proper quantity of fuel at the proper time where compressionignition is used, the machine stalls. In conventional crankshaftmachines the fiy wheel and rotating system store sufficient energy tokeep the machine operating in spite of an occasional misfire, but infree piston units no such energy storage has been available.

Two classes of free piston machines are known at the present time. Inone class the entire work developed in the power cylinder is absorbed inbounce chambers and the work of compression of scavenging air and gasfor external use is developed on the inward stroke of the pistons. Inmachines of this class the front faces of the bounce pistons are used tocompress air or. gas and the work thereof is largely extracted from thesystem through the power cylinder.

In the second class of machines, only a portion of the power cylinderwork is stored in bounce chambers, the remainder being extracted in theform of compressed scavenging air or gas for external use. In asubdivision of this class there are machines in which the bounce energyis made to exceed that required to return the pistons to firingposition, so that a portion of the energy is stored in reverse bouncechambers, to reenter the cycle on the next outward stroke.

It has been proposed to utilize the inner sides of the compressorpistons as reverse bounce spaces for the purpose of maintaining anoutward pressure on the piston system to compensate for changes instroke resulting from changes in atmospheric pressure. It has also beenproposed to utilize this reverse bounce space as a means of increasingthe cyclic frequency of operation. To our knowledge, however, it hasnever been proposed to use this space, or any similar space, in a freepiston machine in combination with other energy storing chambers in sucha manner that the engine will continue to operate in the event of amisfire. To do this requires that the energy contribution of the powercylinder be normally a relatively small part (preferably in the order of24%) of the energy absorbed and released from the bounce space. This maybe done by absorbing bounce chamber energy in a chamber that is in aposition to contribute a substantial or major part of the energyrequired for an outward stroke. If the power cylinder compression work,the scavenging work and friction are.

only a small fraction of the maximum potential energy storedup in thebounce chambers, an inward stroke of the pistons will be made and normaloscillation under only slightly reduced amplitude will continue evenafter a misfire at full load, since a very substantial part of thepotential energy is absorbed in the chamber contributing to the outwardpiston stroke.

A preferred manner of carrying out the invention has been disclosed inthe accompanying drawings, in which:

Fig. 1 is a diagrammatic cross-sectional view of a free piston machineincorporating the present invention; and

Fig. 2 is a graph of piston stroke against accumulated bounce energy.

Referring to the drawings, 1 designates a central power cylinder of thefree piston machine. At each side of the power cylinder are largercylinders 3 and 5, divided into front and rear spaces by the pistonstherein. The cylinder 3 is closed at each end and is used solely for theaccumulation of energy in chambers 4 and 6, while the cylinder 5 actsnot only as an energy accumulator in its rear chamber 7, but also as ascavenging air pumping cylinder 8. To this end the cylinder 5 is fittedwith intake valves 9 at one end and discharge valves 10 intermediate theends, as indicated diagrammatically.

The several cylinders are fitted with opposed piston sets in the usualmanner, each comprising a power piston section 11 and an integral largerpiston 12. Thus, the large piston 12 working in cylinder 3, whichdivides the cylinder into front and rear chambers, compresses air onboth its inward and outward strokes and the energy stored in the air socompressed is returned to the piston by reexpansion on the nextsucceeding stroke as hereinafter described. The large piston working incylinder 5 compresses air during its outward stroke in the outer chamber7, the energy so accumulated being used to assist in the next succeedinginward stroke. During the outward stroke air is drawn into chamber 8behind the piston 12 through the intake valves 9 and is dischargedduring the first portion of the next inward stroke through dischargevalves 10. Discharge of the air so compressed in the scavenging cylinderis stopped when piston 12 overruns the discharge valves 10 andthereafter the scavenging cylinder or inner chamber 8 formed in thiscylinder also acts to accumulate energy.

The discharge ports 10 in the wall of the scavenging cylindercommunicate via a conduit 16 with piston controlled intake ports 15 inthe power cylinder 1, and at the opposite end of the power cylinderthere are the usual exhaust ports 17, also piston controlled.

The machine is ideally suited to use natural or artificial gas as afuel, and a gas injection valve 19 of any suitable. form is interposedin conduit 16. Gas is supplied to a passage 18 controlled by valve 19under pressure higher than that existing in conduit 16 so that, whenvalve 19.

is opened, gas will flow into the conduit and will be.

of the machine. The synchronizing pinion 24 is mounted" on a shaft 25which drives (through appropriate mechanical connections not shown) thegas valve 26 and a magneto 26.

diagrammatically at 27.

The bounce chambers or energy accumulating spaces 4 and 7 at the outsideof the large pistons 12 are interconnected by a bounce equalizing pipe28. The effective clearance adjustment in each of the bounce cylindersmay be adjusted by any suitable means such as a piston 30 If magnetoignition is used the energy from' this unit is taken to a conventionalspark plus indicated.

working in a small cylinder 31 which is, in effect, an extension of therespective bounce chamber volume. The adjustment of the small piston inthese cylinders may be made manual or automatic, depending on the designof the machine.

Provision is made for all of the energy accumulating chambers 4, 6, 7and 8 to operate with atmospheric minimum pressure. This may beconveniently accomplished by using breathing ports. In the case ofbounce chamber 7 the port 50 is formed in a tubular member 51 whichextends from the outside of the machine through piston 22 and is openedby being overrun by the piston. An appropriate check valve prevents airflowing out through the port 50 from the scavenging cylinder 8. In thecase of the bounce cylinder 3, which acts only to accumulate energy onboth strokes of its piston, the front chamber 6 is con nected toatmosphere through a hollow rod 52 having a breathing port 53 thereinwhich is opened to atmosphere at the normal outermost position of piston12 and is closed just after the piston begins its inward stroke. Therear bounce chamber 4 is equalized to chamber 7 (which breathes throughport 50) through the equalizing line 28. In each instance the breathingports 50 and 53 are of a longitudinal extent that corresponds with abouthalf the total expected variations in the normal stroke of the pistons.For example, if the normal variation of stroke amounts to i l, thelongitudinal dimension of the ports will be 1 inch.

The machine shown in the drawing is connected to operate areciprocating, double-acting gas compressor which is indicateddiagrammatically at the left side of the drawing and is designatedgenerally 40. The compressor comprises a cylinder 41 having a compressorpiston 42 operating therein, being directly connected by a tail rod 43to the large piston 12 on the left side of the machine. Intake valves 44and discharge valves 45 are provided at each end of the compressorcylinder so that the driven unit is made double-acting and energy istaken from the free piston system at each inward and outward stroke ofthe machine.

The operation of a machine constructed in accordance with the presentinvention can best be understood by reference to a specific example. Amachine intended to drive a double-acting gas compressor requiring about75 horsepower was designed. The unit was required to operate flexiblythroughout the pressure ranges of: 150 p. s. i. g. to 500 p. s. i. g.;250 p. s. i. g. to 500 p. s. i. g.; 350 p. s. i. g. to 500 p. s. i. g.

The fuel burned was natural gas, so the free piston machine was subjectto the same requirement as to idling conditions that would be expectedof a crank-type gas engine; it must have such stability that occasionalmisfires will not cause it to stall. In a crank-type engine, the kineticenergy stored in a flywheel can be utilized to carry the engine overstalling periods, but in a free piston machine as usually constructedsuch energy storage is not available. The present invention, therefore,introduces a new concept in the construction of free piston engineswhich takes into account for the first time the energy content" ormaximum potential energy that is stored in the bounce chambers at theend of an outward stroke which is deliberately made much larger than theenergy required to return the pistons to center position plus thatrequired to overcome friction and for compression of scavenging air. Ifthe power cylinder work, the scavenging work, the load on the engine andthe work of friction total only a fraction of the potential energy inthe bounce chambers, oscillation of the pistons will continue with onlyslightly reduced amplitude even after a misfire in the power cylinder.In the case described herein by way of example, the free piston machinewill not stall even if the power cylinder misses firing on threesuccessive strokes at idle load or once at full load. Every free pistonmachine with which we are familiar requires faultless ignition of theproper fuel charge at each stroke for continued operation.

Since there is a substantial quantity of energy stored in the bouncechambers over and above that required to return the pistons to centerposition, overcome friction, and product the scavenging work, a verysubstantial storage of energy must take place during the inward stroketo assist the power cylinder in making the following outward stroke.Details of the relative quantities of energy thus stored in the variouschambers is apparent from the following analysis of a typical machine.

The dimensions of the machine are as follows:

Power cylinder: 1

Bore 8".

Stroke 9.5. Scav. port height 2". port height 3". MEP at full load 67.1p. s. i. Pressure at beginning of compression 15.2 p. s. l a.Compression pressure 365 p. s. 1 a.

Scavenging cylinder: 8

Bore 24".

Stroke 9.5". Discharge valve cut-off ratio 2.4. Intake pressure 14.1 p.s. i. a. Discharge pressure 19.7 p. s. i a. Clearance 31.5% P. D.

Bounce cylinders: 4 and 7 Bore 24".

Stroke 9.5. Breathing port height 1". Minimum pressure 14.7 p. s. i. a.(coast). Volumetric compression ratio 3.9.

Idle bounce: 6 (left hand side) Bore 24".

Stroke 9.5". Breathing port height 1". Minimum pressure 14.7 p. s. i. a.(const.) Volumetric compression ratio 3.

Gas compressor: 40

Stroke 6 /2.

Clearance 9.5". Bore P. D.

L. H. I R.H.

Percent Percent to 500 p. s. t. g 34. 4 s2. 4 250 to 500 p. s. i. g 74.4 139.8 350 to 500 p. s. Lg 114 102 .2

Energy balance: (in ft.-lbs.) 250 to 500 p. s. i. g.

Left Side Right Side Inward Outward Inward Outward Power 1 901 4 658-1,991 4,658 7,803 7,268

Friction: -352 ft. lbs. both ways Energy content: 22,646 ft.-lbs.

150 to 500 p. s. i. g.: energy content 22,869 ft.-lbs. 350 to 500 p. s.i. g.: energy content 22,473 ft.-lbs.

The effect of the change in energy content of the bounce cylinders canbe understood with reference to Fig. 2 in the accompanying drawings. InFig. 2 the variation in stroke length of the pistons from the nominal 9/2 inch average in the above machine is plotted against the potentialenergy in each bounce chamber. It

will be understood, of course, that if the pistons make a shorter strokeinto the bounce chambers 4 and 7 the energy accumulated therein isreduced because the ultimate air pressure is lower. Similarly, if thelength of the piston stroke is increased, the air pressure in the energyaccumulating bounce chambers 4 and 7 is increased and the potentialenergy is greater. Fig. 2 shows the relationship between the change instroke length and energy content of the chambers and the 0 linerepresents normal stroke length for operation between 250 p. s. i. g.and 500 p. s. i. g. From 2 it can be seen that:

(1) Operation at full load between 150 and 500 p. s. i. g. increases theouter stroke length by .02 inch because the energy content increasesfrom 22,646 ft. lbs. to 22,867 ft. lbs.

(2) Operation at full load between 350 and 500 p. s. i. g. decreases theouterstroke length by .02 inch because the energy content decreases from22,646 ft. lbs. to 22,473 ft. lbs.

-(3) When operating at full load between 250 and 500 p. s. i. g. onemisfire decreases the outer stroke length by .45" because the energyimparted by the power piston drops from 4,658 ft. lbs. to 1,991 ft. lbs.which represents only the recovery of the energy of compression in thepower cylinder, so that the total energy content is reduced from 22,646ft. lbs. to 19,781 ft. lbs.

(4) A sudden loss of load during an outward stroke which would seriouslydamage most free piston machines results only in an increase in theouter stroke length of .4 because the energy absorbed by the load fromthe left compressor cylinder during the outward stroke is only 2,956 ft.lbs.

'(5) The power cylinder output at each side of the machine is only 11.8%of the energy content of the machine.

(6) Idling conditions result in a reduction in the outer stroke of about.4" While four successive misfires at idling reduces the outer strokelength by .85. The normal contribution of energy by the burning of fuelduring idling is 550 ft. lbs.

(7) The change in the inner position of the pistons is of an ordersiimlar to the change in the outer positions given above, and so long asthe scavenging air and exhaust ports are uncovered in a normal mannerduring the outstroke the engine pistons will continue to reciprocate andthe engine will continue to fire.

It can be seen from the above energy balance that even at full load thepower cylinder output to each side of the machine is only 11.8% of theenergy content of the machine or the maximum potential energy stored inthe bounce chambers 4 and 7. The power cylinder absorbs 1991 2 ft.l=bs., as the work of compression and contributes 4658 2 ft. lbs. asexpansion energy during normal firing at full load operation between asuction pressure of 250 p. s. i. g. of the compressor 40 and a dischargepressure of 500 p. s. i. g. of the compressor. It will also be seen thatthe scavenging cylinder 8 absorbs 7803 ft. lbs. of energy during theinward stroke and contributes 7268 ft. lbs. during the outward stroke toassist in driving the pistons into the bounce chambers 4 and 7. Thedifierence between the energy contained in the inward and outwardstrokes of the scavenging piston in cylinder 8 is taken as the energyrequired to compress the air used for scavenging purposes. Thecompressor, being doubleacting, absorbs 1140 ft. lbs. of energy duringthe inward stroke and 2956 ft. lbs. of energy during the outward stroke.It will be seen that the quantity of gas displaced by the compressorduring the inward stroke of the pistons of the free piston machines isless than the quantity of gas displaced during the outward stroke due tothe presence of the piston rod in the compressor cylinder on the righthand side. The bounce cylinder 6 formed on the inward side of the largepiston in the left hand cylinder absorbs 9369 ft. lbs. of energy duringthe inward stroke and, since the cylinder is closed, contributes thissame quantity of energy during the outward stroke. Totaling the energybalance during the inward stroke of the left hand piston it will be seenthat a total of 12500 ft. lbs. is absorbed during the inward stroke andthat 11323 ft. lbs. of this energy is derivable from the bounce cylinderalone. The deficit in energy is made up by transfer from the right handpiston through the racks 23 and 23a. Totaling the energy in the righthand piston it will be seen that 9794 ft. lbs. of energy is required toforce the pistons into the proper center position and that there are11323 ft. lbs. available for this purpose. There is thus an excess ofenergy in the right hand piston which can be transferred to the lefthand piston and which can also be used for overcoming friction in themachine during the inward stroke.

Similarly, it will be seen that there is an excess of energy during theoutward stroke in the right hand piston and that this is absorbed in theleft hand piston which has a deficit during the outward stroke, thedifference between the two representing the element of friction. Thereis thus energy transferred back and forth in the racks which becomeenergy transmitting elements in addition to functioning for the usualpurpose of synchronizing the motion of the pistons.

It will be seen that the present invention provides, for the first time,a free piston machine that has essentially the stability of a crank-typeengine and can operate satisfactorily on gaseous fuels which are subjectto occasional misfiring. While the invention has been disclosed inconjunction with a specific example it should be appreciated that suchexample is by way of illustration only and that various modificationsand changes in the invention can be made without departing from thescope of the appended claim.

We claim:

The method of controlling the operation of a free piston machine havingpiston controlled exhaust and scavenging ports which comprises, storingand releasing energy in bounce cylinders on an outwardstroke in aquantity at least two times the not quantity of energy derived from thepower cylinder by combustion, and storing and releasing energy in othercylinder spaces in a quantity in excess of that imparted to the pistonsby combustion in the power cylinder, the quantity of energy available tocause an outward stroke of the pistons being at least enough to causemore than one stroke of the pistons of a length sufiicient to uncoversaid piston controlled ports on the outward stroke and to compress acharge to an ignitable point on an inward stroke in the absence of anyenergy imparted by combustion.

References (Iited in the file of this patent UNITED STATES PATENTS2,434,280 Morain Jan. 13, 1948 2,435,970 Lewis Feb. 17, 1948 2,600,251Lewis et al June 10, 1952 2,671,435 Spier ct al. Mar. 9, 1954

